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arXiv 2101.11047astro-ph.SRastro-ph.GA
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盖亚DR2疏散星团中蓝离散星的普查作为布居合成和质量转移物理的检验

A Census of Blue Stragglers in Gaia DR2 Open Clusters as a Test of Population Synthesis and Mass Transfer Physics

Emily M. Leiner, Aaron Geller

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Accepted by ApJ. 27 pages, 15 figures
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摘要：

我们使用测光和自行从盖亚DR 2，以确定16岁（1-10 Gyr），附近（$d< 3500$ pc）疏散星团的蓝离散星（BSS）人口。我们发现，BSS的分数相比，RGB星的年龄增加，开始接近零1 Gyr和平坦的$\sim 0.35$ 4 Gyr。拟合恒星的演化轨迹，这些BSS，我们发现，它们的质量分布峰值在太阳质量的十分之几以上的主序星关闭。高于临界值0.5 $M_\odot$以上的BSS仅占样本的$\sim25$ %，高于临界值1.0 $M_\odot$以上的BSS很少。我们比较这人口合成模型的BSS形成通过质量传递使用紧凑的对象合成和蒙特卡罗调查代码（COSMIC）。我们发现，标准的人口合成的假设显着不足，产生旧的疏散星团的BSS的数量。我们还发现，这些模型过度产生高质量BSS相对于低质量BSS。通过动态形成的BSS的预期数量并不能完全解释这种差异。我们的结论是，为了解释所观察到的BSS人口从罗氏瓣溢出，从巨大的捐助者的传质必须更稳定，比假设在典型的传质处方，包括非保守的传质是很重要的，在生产现实的BSS群众。即使有了这些修改，也很难实现在最古老的疏散星团中观察到的大量BSS。我们讨论了一些额外的物理，可以解释大量的老恒星星族中观察到的蓝离散。

英文摘要：

We use photometry and proper motions from Gaia DR2 to determine the blue straggler star (BSS) populations of 16 old (1-10 Gyr), nearby ($d< 3500$ pc) open clusters. We find that the fractional number of BSS compared to RGB stars increases with age, starting near zero at 1 Gyr and flattening to $\sim 0.35$ by 4 Gyr. Fitting stellar evolutionary tracks to these BSS, we find that their mass distribution peaks at a few tenths of a solar mass above the main-sequence turnoff. BSS more than 0.5 $M_\odot$ above the turnoff make up only $\sim25$\% of the sample, and BSS more than 1.0 $M_\odot$ above the turnoff are rare. We compare this to population synthesis models of BSS formed via mass transfer using the Compact Object Synthesis and Monte Carlo Investigation Code (COSMIC). We find that standard population synthesis assumptions dramatically under-produce the number of BSS in old open clusters. We also find that these models overproduce high mass BSS relative to lower mass BSS. Expected numbers of BSS formed through dynamics do not fully account for this discrepancy. We conclude that in order to explain the observed BSS populations from Roche lobe overflow, mass-transfer from giant donors must be more stable than assumed in canonical mass-transfer prescriptions, and including non-conservative mass transfer is important in producing realistic BSS masses. Even with these modifications, it is difficult to achieve the large number of BSS observed in the oldest open clusters. We discuss some additional physics that may explain the large number of observed blue stragglers among old stellar populations.

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